THE GREAT STAR MAP 13 



so very small as to be negligible, at any rate for the purposes 

 of our star map ; indeed, even in the most minute investi- 

 gations it is easier to neglect the shrinkage as accidental in 

 character than to investigate it. Accidental errors can be 

 obviated by taking another plate (or a number of other plates) 

 and so far as our present experience goes the whole series 

 of plates is very unlikely to be affected by any common or 

 systematic error. Hence the function assigned to the reseau 

 was due to a misapprehension and it has never been used for 

 the purpose originally proposed. Fortunately it has been of 

 immense value in another way. The lines have served as 

 reference marks in determining the places of the stars with 

 facility. To measure the distance between one image and 

 another we might have used a long screw to carry a microscope 

 from one to the other, but it is better to compare the distance 

 with a standard scale, using a screw to connect the stars 

 with the ends of the scale ; the latter method is to be preferred 

 because it avoids the use of a great length of screw. Screws 

 can now be made very accurately if necessary ; Rutherfurd's 

 work laid the foundations of such accurac}^ But they are 

 costly ; their use over a large range takes time in turning the 

 screw through many revolutions ; and continual use is apt to 

 wear away the screw and render it no longer accurate. Hence 

 it is preferable to use the method of comparing with a scale ; 

 the reseau has practically supplied an accurate scale in both 

 directions for the rapid measurement of star positions on the 

 plate. 



We may pause here to remark that the term "map" when 

 applied to the present project must be used in a rather com- 

 prehensive sense. The scheme includes not only the pictorial 

 representations on the plates or on any prints made from them 

 but also the measurement of these plates and the publication 

 of the measures of the individual stars. We can if preferred 

 use a descriptive name for these measures. The printed books 

 containing them are often called the Astrographic Catalogue 

 as opposed to the prints which are the Astrographic Chart 

 proper; but the whole project is really one and the same, 

 although the usual process adopted in making a terrestrial map 

 is here inverted. Surveyors of the face of the earth make 

 careful measurements first and then plot them on a map and 

 that was the method of astronomers before the days of photo- 



